1. Field of the Invention
The present invention relates to frequency selection type high-frequency oscillators that switch between two different oscillating frequencies during operation, and electronic apparatuses including the high-frequency oscillators.
2. Description of the Related Art
In general, a frequency-selecting oscillator which oscillates two high-frequency signals having different frequencies using a single device or circuit is conventionally used in high-frequency oscillator applications, such as a voltage control oscillator (for example, refer to Japanese Unexamined Patent Application Publication No. 2005-57725). Such a frequency-selecting high-frequency oscillator includes a first resonance circuit and a second resonance circuit which resonate at frequencies different from each other, a first amplifier circuit connected to the first resonance circuit, a second amplifier circuit connected to the second resonance circuit, and a selection circuit which selectively activates one of the first and second amplifier circuits. Accordingly, one of the first and second amplifier circuits which are selected using the selection circuit amplifies a resonance signal output from a corresponding one of the first and second resonance circuits, oscillates a high-frequency signal (an output signal) in accordance with the resonance signal, and output the oscillated high-frequency signal.
In the conventional art, the first and second amplifier circuits are implemented using bipolar transistors, both of which are connected to ground using a common capacitor in a high-frequency manner. The common capacitor has an inductance component for a high-frequency signal. Accordingly, to obtain a minimum impedance of the capacitor to the high-frequency signal, a capacitance of the capacitor should be set to be an optimum value in accordance with a frequency of the high-frequency signal. That is, an optimum capacitance of the capacitor varies in accordance with the value of a frequency of the high-frequency signal.
Accordingly, in the conventional art, the capacitance of the grounded capacitor is set to be a value optimum for frequencies of a first and a second high-frequency signal output from the respective first and second amplifier circuits. When the frequencies (oscillating frequencies) of the first and second high-frequency signals have similar values, the optimum capacitances of the capacitor for the first and second high-frequency signals become substantially the same values.
However, if the frequencies of the first and second high-frequency signals have a difference of about 150% or more from each other, the optimum capacitances of the capacitor for the first and second high-frequency signals are different from each other, and accordingly, an oscillating operation of each of the first and second amplifier circuits becomes unstable. Consequently, in a case where a high-frequency oscillator is used as a voltage-control oscillator, for example, a frequency change relative to an output load change is considerably influenced by an unstable oscillating operation.